Fall in love, then become disillusioned (4,5)
I believe the answer is:
lose heart
But I can't explain it at all! Could you help me understand?
(Other definitions for lose heart that I've seen before include "get discouraged?" , "Mislay a card" , "Let hopes dwindle - real ethos involved" , "Here's a lot to cause courage to ebb" , "Become despondent" .)